The present invention relates generally to traveling grate machines and, more particularly to circular traveling grate machines for thermally treating a moving burden, wherein minimum gas leakage is a requirement.
Generally, traveling grate machines are relatively large and are usually designed to handle high tonnage rates of particulate materials in a variety of thermal processes, such as the coking or gasification of coal; heat hardening or prereduction of iron ore pellets; and the retorting of oil shale, to mention a few. The machines are characterized by a plurality of pallets having gas permeable grates, which are moveable along a trackway, supporting a burden through the various zones of the machine. In operation, the pallet grates are first charged with crushed, balled, pelletized or otherwise formed and sized particulate material, such as oil shale, coal, iron ore, or the like. This charged burden is moved slowly by the pallet grates along the trackway through the various process zones. Processes such as coking and oil shale retorting require inert atmospheres and/or atmospheres that contain small controlled amounts of oxygen. The reduction of metallic oxides generally requires an atmosphere containing combustible mixtures of reducing gases. In some of these processes it is necessary, from a safety standpoint, to prevent ambient air leakage in order to guard against uncontrolled burning and/or explosion dangers. Thus, it is necessary to minimize, in so far as possible, leakage of ambient air into (or process gases out of) the apparatus, and, to this end, some form of sealing means around the traveling grate is essential. It is also desirable to isolate the pallet wheels and frame supporting components of the machines from the intense heat of the process zone so as to prolong service life. It is further desirable to employ a pallet design which reliably and quickly discharges its burden. Further, the pallets should be capable of being removed from the machine easily and quickly in the event that maintenance or replacement is required, without the need for expensive down time or elaborate equipment.
Heretofore, various circular traveling grate machines have been proposed to improve the gas sealing ability of the device as well as to improve upon the pallet configuration, its structural supports and the maintenance thereof. Exemplary of such is the circular traveling grate machine disclosed in U.S. Pat. No. 4,408,987 in which the pallet wheels travel in an isolated tunnel chamber filled with an inert gas. This tunnel-like configuration necessarily poses a serious drawback regarding inspection and maintenance of the pallet and side frame wheels since these wheels are hidden from view around most of the machine. Gas sealing of the process zones is achieved through an upper pair of liquid seals and lower mechanical seals in conjunction with the aforementioned tunnels.
U.S. Pat. No. 3,302,936 discloses (in FIGS. 13-15, thereof) a circular traveling grate machine for removing oil from oil bearing shale in a retort process wherein the pallets carry a moving sidewall of the hearth on opposed sides thereof. The marginal edges of the sidewalls engage upper and lower water troughs for sealing purposes. The pallets have a central, third wheel which runs on a dump rail in the center of the pallet trackway and is, thus, continually exposed to excessive heat which necessarily leads to a relatively short work life of the third wheel and its supporting structure. In addition, the outer wheels of the pallet must not only carry the weight of the pallet and that of the burden but they also must support the weight of the vertical sidewalls of the hearth. This objectionable sidewall loading characteristic is also present in the above-discussed U.S. Pat. No. 4,408,987, which necessarily increases the wear factor on the pallet wheels, bearings, and related structural elements, causing increased maintenance and decreased service life.
The present invention solves many of the problems heretofore encountered in the construction, operation and maintenance of such devices by providing a circular traveling grate machine having upper and lower liquid seals which create a hermetically sealed environment in the process zones. The invention further provides for separate sets of load bearing wheels and trackways, a first set for the rotating side frame members of the hearth and a second set for the pallets. The wheeled trucks supporting the side frames are externally accessible for visual inspection and maintenance purposes. Still further, my invention provides a pallet construction which includes a pair of rotatable hanger brackets at the leading end thereof which detachably latch to the underside of the rotating inner and outer side frames. A fast-acting pallet release latch mechanism which permits easy removal of the pallet and replacement thereof is also provided. My invention further provides mechanical seals in the area of the discharge zone adjacent the lower liquid sealing troughs. These seals prevent a standing wave or water fall effect in the lower water seals along the discharge zone which is exposed to a differential in pressure from the process zone. The invention still further provides a plurality of motor assemblies with gear drives which are mounted at spaced intervals around the inner and outer side frames and rotatably engage gear racks integral with the side frames to rotate the side frames around the circular trackway. Each of the drive assemblies are pivoted about a vertical axis to accommodate thermal expansion while maintaining engagement with the gear rack. Also provided are a plurality of anchored, guide rollers which engage a ring-shaped rail, mounted on the inner side frame to maintain the circular concentricity of the traveling grate machine.